PHASE DIFFERENCE CALCULATION DEVICE, PHASE DIFFERENCE CALCULATION METHOD, AND PROGRAM

Abstract

 Provided is a phase difference calculation device including a first light amount acquisition unit that acquires a first light amount of reflected light of light applied in a first time window and received in the first time window and a second light amount of the reflected light received in a second time window, a time window shift control unit that shifts the first and second time windows and a third time window in the negative direction of the time axis to set fourth, fifth, and sixth time windows, and shifts the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window, a second light amount acquisition unit that acquires a third light amount of the reflected light received in the sixth time window, and a phase difference calculation unit that calculates a phase difference between the light and the reflected light on the basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

Description

[Technical Field]

[0001] The present invention relates to a phase difference calculation device, a phase difference calculation method, and a program.

[Background Art]

[0002] A ToF (Time of Flight) sensor that measures a distance on the basis of the time of flight of light is used in order to acquire, for example, three-dimensional information of a subject. A technique related to the ToF sensor is described in, for example, PTL 1. In the technique described in PTL 1, a depth image acquisition device includes a light emitting diode for applying modulated light toward a detection region, a ToF sensor for receiving light that is reflected by an object in the detection region and that enters the ToF sensor and outputting a signal for generating a depth image, and a filter for allowing, among the incident light, relatively more light having a predetermined wavelength band to pass through, and at least any one of the arrangements of the light emitting diode, the ToF sensor, and the filter is controlled according to the temperature of the light emitting diode or the ToF sensor.

[Citation List]

[Patent Literature]

[0003] [PTL 1]
Japanese Patent Laid-open No. 2019-078748

[Summary]

[Technical Problem]

[0004]  However, in a case where the ToF sensor is used in an environment where other objects are present around the object, such as an indoor room, there is a possibility that light reflected by the same object enters the ToF sensor through a plurality of reflection paths and the measurement accuracy is thus reduced. A technique for coping with what is generally called multipath reflected light as described above is not described in, for example, PTL 1.

[0005] Accordingly, an object of the present invention is to provide a phase difference calculation device, a phase difference calculation method, and a program that can improve the measurement accuracy of a ToF sensor by reducing the influence of multipath reflected light.

[Solution to Problem]

[0006] According to an aspect of the present invention, provided is a phase difference calculation device including a first light amount acquisition unit that, among continuous first, second, and third time windows having the same length, acquires a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window, a time window shift control unit that shifts the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows, and shifts the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window, a second light amount acquisition unit that acquires a third light amount of the reflected light received in the sixth time window, and a phase difference calculation unit that calculates a phase difference between the light and the reflected light on the basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

[0007] According to another aspect of the present invention, provided is a phase difference calculation method including a step of, among continuous first, second, and third time windows having the same length, acquiring a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window, a step of shifting the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows, and shifting the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window, a step of acquiring a third light amount of the reflected light received in the sixth time window, and a step of calculating a phase difference between the light and the reflected light on the basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

[0008] According to still another aspect of the present invention, provided is a program that causes a computer to realize a function of, among continuous first, second, and third time windows having the same length, acquiring a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window, a function of shifting the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows by a time window shift control unit, and shifting the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window, a function of acquiring a third light amount of the reflected light received in the sixth time window, and a function of calculating a phase difference between the light and the reflected light on the basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

[Brief Description of Drawings]

[0009] 

[FIG. 1]
FIG. 1 is a diagram for explaining an outline of measurement by a ToF sensor according to an embodiment of the present invention.

[FIG. 2]
FIG. 2 is a first timing chart for depicting an example of an operation of the ToF sensor according to the embodiment of the present invention.

[FIG. 3]
FIG. 3 is a second timing chart for depicting an example of an operation of the ToF sensor according to the embodiment of the present invention.

[FIG. 4]
FIG. 4 is a diagram for explaining the principle of removing or reducing the influence of multipath reflected light in the embodiment of the present invention.

[FIG. 5]
FIG. 5 is a diagram for depicting a schematic configuration of the ToF sensor according to the embodiment of the present invention.

[FIG. 6]
FIG. 6 is a diagram for depicting a circuit configuration example of a light reception unit applicable to the ToF sensor depicted in FIG. 5.

[FIG. 7]
FIG. 7 is a flowchart for depicting schematic steps of a phase difference calculation method according to the embodiment of the present invention.

[Description of Embodiment]

[0010] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that in the specification and the drawings, constituent elements having substantially the same functional configurations will be denoted by the same reference signs, and duplicate explanations thereof will be omitted.

[0011] FIG. 1 is a diagram for explaining an outline of measurement by a ToF sensor according to the embodiment of the present invention. A ToF sensor 100 applies pulse light PL from a light source. Reflected light of the pulse light PL, the reflected light being reflected by an object obj, is received by a light reception unit of the ToF sensor 100. A distance d from the ToF sensor 100 to the object obj can be calculated on the basis of a phase difference between the pulse light PL and the reflected light. However, the reflected light received by the ToF sensor 100 includes, other than reflected light RL of the pulse light PL, the reflected light RL being reflected only by the object obj, for example, multipath reflected light MRL1 to MRL3 of the pulse light PL, the multipath reflected light MRL1 to MRL3 being reflected by a ceiling or a floor and the object obj or by both the ceiling and the floor and the object obj. Since the phase difference between the reflected light RL and the pulse light PL is different from that between the multipath reflected light MRL1 to MRL3 and the pulse light PL, it becomes difficult to calculate the accurate distance d if the multipath reflected light MRL1 to MRL3 are mixed in the reflected light RL.

[0012] FIG. 2 is a first timing chart for depicting an example of an operation of the ToF sensor according to the embodiment of the present invention. In the illustrated example, in the ToF sensor 100, light is received in each of continuous time windows TW1 to TW3 having the same length by using a shutter S1 that opens in synchronization with irradiation time r of the pulse light PL applied from the light source, a shutter S2 that opens only for the same length as the irradiation time r from the end of the application of the pulse light PL, and a shutter S3 that opens only for the same length as the irradiation time r from the time when the shutter S2 closes. The pulse light PL is applied in the time window TW1, and the reflected light of the pulse light PL applied in the initial stage of the irradiation time r is received within the time window TW1. The light amount of the reflected light received in the time window TW1 is depicted as a light amount Q1. Meanwhile, the reflected light of the pulse light PL applied in the later stage of the irradiation time r is received in the time window TW2. The light amount of the reflected light received in the time window TW2 is depicted as a light amount Q2. It should be noted that the light amount of light to be received includes the light amount of background light BG, but, for example, the light amounts Q1 and Q2 of the reflected light excluding the influence of the background light BG can be calculated by subtracting the light amount of light received in the time window TW3 where no reflected light is received from the light amounts of the time windows TW1 and TW2 as the light amount of the background light BG.

[0013]  As the distance d to the object obj described above with reference to FIG. 1 becomes larger, the phase delay of the reflected light relative to the pulse light PL becomes larger, and therefore, the ratio of the light amount Q1 becomes smaller and the ratio of the light amount Q2 becomes larger. By using this, the ToF sensor 100 calculates the phase difference between the pulse light PL and the reflected light from the ratio of the light amount Q1 to the light amount Q2, and further calculates the distance d from the phase difference. However, if the multipath reflected light MRL1 to MRL3 to be received later than the reflected light RL are mixed, the accuracy of the phase difference and the distance d to be calculated is reduced. Specifically, the inclusion of the multipath reflected light MRL1 to MRL3 causes a deviation between the ratio of the light amount for each of the time windows TW1 and TW2 in the reflected light RL correctly reflecting the distance d and the ratios of the light amounts Q1 and Q2 of the actually observed reflected light. Accordingly, in the present embodiment, the influence of the multipath reflected light in the light amount of the reflected light received in each time window is reduced by additionally executing the procedure described below.

[0014] FIG. 3 is a second timing chart for depicting an example of an operation of the ToF sensor according to the embodiment of the present invention. In the illustrated example, the timings of the shutters S1 to S3 are advanced while the application of the pulse light PL in the time window TW1 depicted in FIG. 2 is maintained. Thus, the original time windows TW1 to TW3 are shifted in the negative direction of the time axis to become time windows TW4 to TW6. The time windows TW4 to TW6 are shifted until no reflected light is received in the time window TW4, as illustrated in FIG. 3. In this case, the timing at which the reception of the reflected light RL reflected only by the object obj starts matches the end point of the time window TW4, that is, the start point of the time window TW5. Since the reflected light RL is received only for the same length as the irradiation time r of the pulse light PL, the timing at which the reception of the reflected light RL ends matches the end point of the time window TW5.

[0015] On the other hand, since the multipath reflected light MRL1 to MRL3 are received later than the reflected light RL as described above, the timing at which the reception of the multipath reflected light MRL1 to MRL3 starts does not match the start point of the time window TW5, and the timing at which the light reception ends does not match the end point of the time window TW5. As a result, the multipath reflected light MRL1 to MRL3 are also received after the end point of time window TW5, that is, in the time window TW6. That is, the light amount Q3 of the reflected light received in the time window TW6 does not include the light amount of the reflected light RL, and includes only the light amounts of the multipath reflected light MRL1 to MRL3.

[0016] FIG. 4 is a diagram for explaining the principle of removing or reducing the influence of the multipath reflected light in the embodiment of the present invention. As described above, in a case where the time windows TW4 to TW6 are shifted until the light amount of the reflected light received in the time window TW4 becomes substantially zero, the light amount Q3 of the reflected light received in the time window TW6 does not include the light amount of the reflected light RL, and includes only the light amounts of the multipath reflected light MRL1 to MRL3. Therefore, as illustrated in FIG. 4, when the light amount Q3 is added to the light amount Q1 of the reflected light in the time window TW1 to be a corrected light amount Qm1 and the light amount Q3 is subtracted from the light amount Q2 of the reflected light in the time window TW2 to be a corrected light amount Qm2, the ratios of the corrected light amounts Qm1 and Qm2 become the same as the ratio of the light amount for each of the time windows TW1 and TW2 in the reflected light RL that does not include the multipath reflected light. By calculating the phase difference and the distance d on the basis of the corrected light amounts Qm1 and Qm2 described above, the influence of the multipath reflected light can be reduced, and the measurement accuracy of the ToF sensor 100 can be improved.

[0017] FIG. 5 is a diagram for depicting a schematic configuration of the ToF sensor according to the embodiment of the present invention. In the illustrated example, the ToF sensor 100 includes a light source unit 110, a light reception unit 120, and a control unit 130. The light source unit 110 includes, for example, an infrared laser light source and applies the above-described pulse light PL. The light reception unit 120 includes, for example, a light reception element such as a photodiode and the shutters S1 to S3 arranged for each pixel of the light reception element to block light incident on the light reception element at predetermined time intervals, and receives the reflected light of the pulse light PL in a predetermined time window.

[0018] The control unit 130 is implemented by a computer having, for example, a communication interface, a processor, and a memory. The functions of a light source control unit 131, a shutter timing control unit 132, a first light amount acquisition unit 133, a time window shift control unit 134, a second light amount acquisition unit 135, a phase difference calculation unit 136, and a distance calculation unit 137 are realized in software by an operation of the processor according to a program stored in the memory or received via the communication interface. Hereinafter, the function of each unit will be described.

[0019] The light source control unit 131 controls the application timing of the pulse light PL in the light source unit 110. Specifically, the light source control unit 131 performs control such that the pulse light PL is applied only for the irradiation time r in a predetermined cycle. In the illustrated example, the shutter timing control unit 132 controls the light source unit 110 such that the irradiation time of the pulse light PL matches the time window TW1 depicted in FIG. 2.

[0020] The shutter timing control unit 132 controls the opening/closing timings of the shutters S1 to S3 in the light reception unit 120. The shift of the time windows TW1 to TW3 to the time windows TW4 to TW6 depicted in FIG. 2 and FIG. 3 is executed in such a manner that the time window shift control unit 134 inputs the change amounts of the shutter timings to the shutter timing control unit 132.

[0021] The first light amount acquisition unit 133 acquires the light amounts Q1 and Q2 in which the reflected light of the pulse light PL applied in the time window TW1 and received in the time windows TW1 and TW2, respectively. Here, the computation for removing the influence of the background light BG described above with reference to FIG. 2 may be executed by the first light amount acquisition unit 133 or may be executed by the phase difference calculation unit 136 to be described later. As described with reference to FIG. 2, the light amounts Q1 and Q2 include the light amounts of the reflected light RL reflected by the object obj only and the multipath reflected light MRL1 to MRL3.

[0022]  The time window shift control unit 134 controls the shift of the time windows TW1 to TW3 to the time windows TW4 to TW6 depicted in FIG. 2 and FIG. 3. Specifically, after the first light amount acquisition unit 133 acquires the light amounts Q1 and Q2, the time window shift control unit 134 inputs the change amounts of the shutter timings to the shutter timing control unit 132 such that the operation timings of the shutters S1, S2, and S3 are advanced only by a time corresponding to the shift amounts of the time windows TW1 to TW3 in the negative direction of the time axis.

[0023] In the present embodiment, the time window shift control unit 134 acquires, from the light reception unit 120, a comparison result of the light amounts of the reflected light of the pulse light PL applied after the change of the shutter timings and received in the time windows TW4 to TW6, or a light amount as described later. In a case where it is determined that the reflected light is no longer received in the time window TW4 on the basis of the acquired light amount or the comparison result, the time window shift control unit 134 terminates the shift of the time windows and decides the time windows TW4 to TW6.

[0024] The second light amount acquisition unit 135 acquires the light amount Q3 of the reflected light of the pulse light PL applied in the time window TW1 and received in the time window TW6. As described above, even if the shutter timings are shifted according to the time windows TW4 to TW6, the application of the pulse light PL is held in the time window TW1. As described with reference to FIG. 4, the light amount Q3 corresponds to the light amount of at least any of the multipath reflected light MRL1 to MRL3 received later than the reflected light RL.

[0025]  The phase difference calculation unit 136 calculates the phase difference between the pulse light PL and the reflected light on the basis of the corrected light amount Qm1 obtained by adding the light amount Q3 acquired by the second light amount acquisition unit 135 to the light amount Q1 acquired by the first light amount acquisition unit 133 and the corrected light amount Qm2 obtained similarly by subtracting the light amount Q3 from the light amount Q2. The distance calculation unit 137 calculates the distance d on the basis of the phase difference. It should be noted that the calculation of the phase difference after the correction of the light amount and the calculation of the distance based on the phase difference can be performed by using a known technique, and thus the detailed description thereof will be omitted.

[0026] According to the ToF sensor 100 having the above-described configuration, the accuracy of measurement of the distance d can be improved by reducing the influence of the multipath reflected light in the calculation of the phase difference. It should be noted that the ToF sensor 100 may be mounted as an integral device including the configuration depicted above in FIG. 5, or may be mounted as a distributed device. In this case, if the light source unit 110 and the light reception unit 120 are arranged close to each other, the control unit 130 need not necessarily be mounted in the same device as the light source unit 110 and the light reception unit 120. In addition, the constituent elements of the control unit 130 need not necessarily be mounted in a single device, and, for example, the light source control unit 131 may be incorporated into a light source device including the light source unit 110, the shutter timing control unit 132 may be incorporated into a light reception device including the light reception unit 120, and the first light amount acquisition unit 133, the time window shift control unit 134, the second light amount acquisition unit 135, and the phase difference calculation unit 136 may be mounted as a phase difference calculation device different therefrom. For example, the distance calculation unit 137 may be mounted as another device for calculating the distance d on the basis of the phase difference output by the phase difference calculation device. The above-described ToF sensor 100 is also an example of the phase difference calculation device including a configuration for calculating the phase difference.

[0027] FIG. 6 is a diagram for depicting a circuit configuration example of the light reception unit applicable to the ToF sensor depicted in FIG. 5. In the illustrated example, the circuit configuration includes a photodiode PD, capacitors C1 and C2, a reset switch RSW, an amplifier AMP, a switching switch SSW, a comparator CMP, and a switch SW, and outputs a pixel signal SIG and comparator output CMP-OUT. The photodiode PD is arranged corresponding to a pixel in which the shutter S1 is arranged in the light reception unit 120, and detects a light amount of light incident on the pixel. Before the shutter timing control unit 132 to which the change amounts of the shutter timings are input from the time window shift control unit 134 changes the shutter timings, the switching switch SSW is connected to the capacitor C2 side, and an electric charge corresponding to the light amount of light received in the time window TW1 that has not yet undergone the shift or the time window TW4 is charged to the capacitor C2.

[0028] Thereafter, after the shutter timing control unit 132 changes the shutter timings, the electric charge of the capacitor C1 is reset by the reset switch RSW, the switching switch SSW is then connected to the comparator CMP side, and the electric charge corresponding to the light amount of light received in the time window TW4 that has undergone the shift is compared with the electric charge that is charged to the capacitor C2 and that corresponds to the light amount that has not yet undergone the shift. In a case where the output of the comparator CMP indicates that the electric charge after the shift is smaller than the electric charge before the shift, the time window shift control unit 134 further shifts the time windows TW4 to TW6 in the negative direction of the time axis. In a case where the output of the comparator CMP indicates that the electric charge before the shift and the electric charge after the shift are the same, the time window shift control unit 134 terminates the shift of the time windows and decides the time windows TW4 to TW6. The time window shift control unit 134 may set, to the shift amounts of the time windows TW4 to TW6, the last shift amounts indicating that the electric charge after the shift is smaller than the electric charge before the shift.

[0029] As described above, a circuit configuration capable of comparing the light amounts received before and after the shift of the time windows may be provided for each pixel of the light reception unit 120, and the time window shift control unit 134 of the control unit 130 may determine the termination of the shift of the time windows due to the change of the shutter timings, on the basis of the comparison result (comparator output CMP-OUT) of the light amounts. Alternatively, the light reception unit 120 may not be provided with the above-described circuit configuration, and the time window shift control unit 134 of the control unit 130 may determine the termination of the shift of the time windows by comparing the light amount received in the time window TW1 that has not yet undergone the shift and the light amount received in the time window TW4 that has undergone the shift. In the above example, the termination of the shift is determined on the basis of the result of the comparison between the light amount received in the time window TW1 that has not yet undergone the shift and the light amount received in the time window TW4 that has undergone the shift, but in another example, the shift of the time windows may be terminated in a case where the light amount received in the time window TW4 becomes the same as the light amount (light amount of the background light BG) received in the time window TW3.

[0030] FIG. 7 is a flowchart for depicting schematic steps of a phase difference calculation method according to the embodiment of the present invention. In the illustrated example, each step is executed by the control unit 130 of the ToF sensor 100. When the pulse light PL is applied under the control of the light source control unit 131 and the shutter timing control unit 132 controls the shutters S1 to S3 such that the reflected light is received in the time windows TW1 to TW3, the first light amount acquisition unit 133 acquires the light amounts Q1 and Q2 of the time windows TW1 and TW2 (Step S101). As described above, the light amounts Q1 and Q2 acquired here include the light amounts of the reflected light RL and the multipath reflected light MRL1 to MRL3. Next, the time window shift control unit 134 shifts the time windows TW1, TW2, and TW3 in the negative direction of the time axis (Step S102). For example, the time window shift control unit 134 repeats the shift of the time windows by predetermined amounts until no reflected light is received in the time window TW4 (Step S103) .

[0031] After the shift of the time windows is terminated, the second light amount acquisition unit 135 acquires the light amount Q3 of the time window TW6 (Step S104). The phase difference calculation unit 136 calculates the phase difference between the pulse light PL and the reflected light on the basis of the corrected light amount Qm1 obtained by adding the light amount Q3 to the light amount Q1 and the corrected light amount Qm2 obtained by subtracting the light amount Q3 from the light amount Q2 (Step S105). The calculated phase difference may be used for calculating the distance d in the distance calculation unit 137, or may be stored or output as a phase difference and used for calculating the distance d afterwards or by another device.

[0032] Although the preferred embodiment of the present invention has been described above in detail with reference to the accompanying drawings, the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention belongs can arrive at various examples of changes or modifications within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical range of the present invention.

[Reference Signs List]

[0033] 

100: ToF sensor

110: Light source unit

120: Light reception unit

130: Control unit

131: Light source control unit

132: Shutter timing control unit

133: First light amount acquisition unit

134: Time window shift control unit

135: Second light amount acquisition unit

136: Phase difference calculation unit

137: Distance calculation unit

PL: Pulse light

RL: Reflected light

MRL1 to MRL3: Multipath reflected light

Q1 to Q3: Light amount

Qm1, Qm2: Corrected light amount

S1 to S3: Shutter

TW1 to TW6: Time window

Claims

1. A phase difference calculation device comprising:

a first light amount acquisition unit that, among continuous first, second, and third time windows having a same length,

acquires a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window;

a time window shift control unit that shifts the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows, and shifts the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window;

a second light amount acquisition unit that acquires a third light amount of the reflected light received in the sixth time window; and

a phase difference calculation unit that calculates a phase difference between the light and the reflected light on a basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

2. The phase difference calculation device according to claim 1, further comprising:

a light reception unit that includes a light reception element for receiving the reflected light and a shutter for blocking light incident on the light reception element at predetermined time intervals,

wherein the time window shift control unit shifts the fourth, fifth, and sixth time windows in the negative direction of the time axis by advancing an operation timing of the shutter.

3. The phase difference calculation device according to claim 2, wherein the light reception unit further includes a circuit configuration capable of comparing light amounts received in the fourth time window, the light amounts being received before and after the operation timing of the shutter is advanced.

4. A phase difference calculation method comprising:

a step of, among continuous first, second, and third time windows having a same length, acquiring a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window;

a step of shifting the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows, and shifting the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window;

a step of acquiring a third light amount of the reflected light received in the sixth time window; and

a step of calculating a phase difference between the light and the reflected light on a basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

5. A program that causes a computer to realize:

a function of, among continuous first, second, and third time windows having a same length, acquiring a first light amount of reflected light of light applied in the first time window and received in the first time window and a second light amount of the reflected light received in the second time window;

a function of shifting the first, second, and third time windows in a negative direction of a time axis to set fourth, fifth, and sixth time windows by a time window shift control unit, and shifting the fourth, fifth, and sixth time windows in the negative direction of the time axis until no reflected light is received in the fourth time window;

a function of acquiring a third light amount of the reflected light received in the sixth time window; and

a function of calculating a phase difference between the light and the reflected light on a basis of a first corrected light amount obtained by adding the third light amount to the first light amount and a second corrected light amount obtained by subtracting the third light amount from the second light amount.

Drawing